isPending defers superseded async-iterable flight teardown (stale iterator runs to completion) #3122

Summary

When a computation's async-iterable flight is superseded, the stale iterator is normally closed (it.return()) synchronously at recompute. Adding a single isPending read over the same source changes this: the superseded iterator is left running — it can land a value that flight identity then discards — and return() only fires after the superseding flight settles.

Iterator close is the cancellation hook for anything resource-shaped behind an async iterable (fibers, sockets, subscriptions), so the deferral means wasted in-flight work and late resource teardown. Found while wiring Effect fibers to memos (examples/effect): the superseded search fiber keeps running its retries to completion instead of being interrupted at supersede time.

Ablation

Same graph in all four cases; only the reader changes. Memo flips [] → 400ms async iterable per query write; supersede happens 100ms in.

Observed output for the failing case:

    1ms  open  a
  103ms  write 'ab' (supersedes 'a')
  103ms  open  ab
  402ms  land  a      <-- stale flight ran to completion (value discarded)
  505ms  land  ab
  505ms  close a      <-- return() only after 'ab' settled

Expected (and observed as soon as the isPending read is removed):

    1ms  open  a
  103ms  write 'ab' (supersedes 'a')
  103ms  open  ab
  103ms  close a      <-- eager close at supersede
  504ms  land  ab

In the full web-render structure (examples/effect typeahead) the deferral only manifests on the first flight after a region mounts; in this minimal graph it defers every supersede.

Repro

Checked in as examples/effect/repro-close-timing.mjs (runs against packages/signals/dist/dev.js, no external deps). Toggle the isPending line inside the boundary content to flip between eager and deferred close:

const view = createLoadingBoundary(
  () => {
    try {
      // Comment out this isPending read → closes become eager again.
      const pending = isPending(() => m());
      return { pending, value: latest(() => m()) };
    } catch {
      return undefined;
    }
  },
  () => "loading-fallback"
);

Run with node examples/effect/repro-close-timing.mjs.

Environment

• next (2.0.0-rc.x), observed at 42f2e8d
• pnpm 11.1.1, Node 26.4.0, macOS